Mesomorphic or liquid crystal compositions are of increasing interest in a variety of electro-optic display devices. Liquid crystal compositions are of particular interest for electrically controllable, flat panel displays such as watch faces, digital clocks, calculator displays, numeric displays for instruments and the like. An electro-optic device, e.g., a liquid crystal cell, comprises a layer of a liquid crystal composition between two closely spaced parallel conductive plates, at least one of which is transparent. When the conductive plates are connected to a source of voltage, an electric field is generated in the liquid crystal composition.
Field effect liquid crystal devices contain nematic compounds or mixtures of liquid crystal compounds having positive dielectric anisotropy. The conductive plates have been treated so that the liquid crystal molecules align themselves in a particular direction, usually parallel, to the plane of the plates. When an electric field is applied, the positive dielectric anisotropy of the molecules causes them to realign themselves in a direction parallel to the applied field and perpendicular to the plates. The change in alignment is made visible using a polarizer and an analyzer on either side of the cell. Field effect liquid crystal cells have the advantages of lower threshold voltages and wider viewing angle than other electro-optic devices such as dynamic scattering cells, and they have excellent contrast and long lifetimes.
U.S. Pat. No. to Gavrilovic, 3,951,846, incorporated herein by reference, discloses liquid crystal compounds having high and broad use temperature ranges which have the formula ##STR4## wherein X can be hydrogen, alkyl, alkoxy, acyloxy or alkylcarbonato wherein the alkyl groups have 1- 10 carbon atoms. These liquid crystal compounds have positive dielectric anisotropy and are useful in field effect liquid crystal devices.
Each mesomorphic compound has a particular temperature range in which it is an ordered liquid, ranging from the solid to nematic liquid crystal melting point, up to the temperature at which it forms an isotropic liquid. This is the temperature range useful in electro-optic cells. Although, as is known, wide variations in use temperature ranges can be effected by employing mixtures of liquid crystal compounds that are compatible with each other rather than single compounds, no single liquid crystal compound or mixture of compounds now known can satisfy all of the use temperature ranges that are desired. The particular mesomorphic temperature range for each compound or even whether a compound will be mesomorphic or not is on the whole unpredictable. Thus, new liquid crystal compounds which have different use temperature ranges are being sought to satisfy various temperature requirements for which the liquid crystal cells will be employed.